The present invention relates to in situ mining of metal values; and, more particularly, to a downhole device for recirculating lixiviant in a leaching interval.
The subject matter of the present application is related to the subject matter of patent application Ser. No. 724,548, filed Sept. 20, 1976, entitled "In-Situ Method and Apparatus," and that application is incorporated by reference in the present application.
As noted in the incorporated reference, much contemporary effort is directed to the development of processes and hardware permitting the efficient and economic extraction of metal values from low grade porphyry ores residing in relatively large deep-lying deposits, whereby the metal value extraction may be accomplished with minimal environmental impact.
Although in situ mining can be with a single hole generally, for economic consideration, in situ mining processes require at least two bore holes drilled to the lowermost level of the desired leaching interval in the ore deposit. A packer and lixiviant injector is then affixed to the interior of a first, or injection, hole at the top of the desired leaching interval. Leach liquor is pumped down the injection hole and into the leaching interval to establish a relatively high pressure reservoir of leach liquor in the portion of the injection hole selected as the leaching interval. A relatively low pressure is established in one or more nearby production holes at portions of those holes lying within the leaching interval. Lixiviant from the injection hole reservoir passes through fissures in the ore along a pressure gradient between the injection hole and the production holes. As the lixiviant passes through the ore, metal values are leached. The pregnant leach liquor is pumped to the surface by way of the production holes and processed to recover the metal values.
The broad concept of leaching metal values from ore formations in situ is known in the art. However, the known techniques have proven to be effective only for shallow ore deposits where fractures are relatively large, or for deep deposits where the ore has first been rubblized to establish relatively large passages for the lixiviant.
The incorporated reference discloses an in situ deep mining technique whereby the metal values such as nickel and copper, may be recovered from deep, hard rock formations characterized by low porosity. For example, using the latter technique, copper can be recovered from ore deposits having fractures on the order of 30-300 microns in width utilizing a lixiviant which contains very small oxygen bubbles admixed with a leach liquor, wherein the oxygen bubbles are smaller than fractures residing in the ore. As a result, there is no requirement for rubblizing the ore prior to leaching.
However, as disclosed in the incorporated reference, in order to maintain a two phase lixiviant solution with sufficiently small oxygen bubbles, the lixiviant must move continuously at a velocity above a predetermined value within the injection hole. Accordingly, the referenced disclosure teaches the use of a device for recirculating the injection hole lixiviant within a leaching interval wherein the injection hole lixiviant is injected into the leaching interval through a venturi-type exhauster having an extended injection nozzle (also called stinger or tail pipe) which is downwardly directed and terminated within the leaching interval. The two phase solution of lixiviant and oxygen bubbles is maintained by establishing a continuous vertical circulation of lixiviant between the outlet of the injection nozzle (located in the lower portion of the leaching interval) and the recirculating device which provides at least one aspirator passage (located in the upper portion of the leaching interval). With this configuration, a primary flow of lixiviant passes from the surface through the injection nozzle. The primary flow induces a relatively low pressure near the aspirator passage. Consequently, the lixiviant ejected at the nozzle tends to flow from that region at the bottom of the leaching interval toward the top of the leaching interval and through the aspirator passage to form a secondary flow into the injection nozzle, thereby achieving a continuous vertical circulation. In addition, oxygen which has come out from the solution and started to accumulate near the top of the leaching interval is also drawn into the aspirator passage and re-introduced to the primary fluid stream.
In the incorporated reference, the exhauster is permanently affixed to the injection hole lixiviant supply tubing string and a sealing packer assembly which defines the upper limit to the leaching interval. While this configuration does generally provide continuous circulation of the injection hole lixiviant sufficient to maintain a two-phase leaching solution, the recirculation device described in that reference does place substantial limits on the long term yields and efficiencies of the in situ mine for a number of reasons. One such reason is that the effectiveness of the recirculation device falls off with time due to the accumulation of debris and mineral deposits within the device. As a result, the lixiviant circulation, while initially satisfactory, degrades over time to a point where the two phase solution may no longer be adequately maintained.
A further limitation of the reference system relates to the flow rate of lixiviant into the injection hole. Since it is known that the two phase solution may only be maintained at velocities above a predetermined value, circumstances which require a lowering of the injection hole velocity into an injection hole may result in lixiviant velocity below the required minimum.
In order to overcome the first of these limitations, a clogged recirculation device may be replaced with another recirculation device which is deposit free. Regarding the latter limitation, a relatively low lixiviant flow rate may still provide satisfactory lixiviant velocity by providing a relatively small diameter injection tube which is coupled to the recirculation device in the injection hole. However, in the configuration disclosed in the incorporated reference, replacement of either the recirculation device or injection tubing string, or both requires removal of the entire injection tubing string and packer assembly, including the recirculation device, with a consequent relatively large expense.
Accordingly, it is an object of the present invention to provide a recirculation device which is readily installed and retrieved from the operating position in a packer assembly of an injection hole of an in situ mining configuration.
A further object is to provide a recirculation device which may accommodate interchanging of the injection tubing string while in its operating position in an injection hole of and in situ mining configuration.